1. Field of the Invention
The present invention relates to a semiconductor device using a thin film transistor (hereinafter referred to as a TFT) wherein a semiconductor film formed on a substrate and having a crystal structure is used; and a process for manufacturing the same. The semiconductor device in the present specification means any device which functions by the use of a semiconductor characteristic, and the category of the semiconductor device manufactured by the present invention includes a display device, a typical example of which is a liquid crystal display device having therein TFTs, and a semiconductor integrated circuit (such as a microprocessor, a signal processing circuit, or a high frequency circuit).
2. Description of Related Art
In various semiconductor devices having therein semiconductor elements, such as a television, a personal computer or a portable telephone, a display for displaying characters or images is an essential means from which people recognize information. Particularly in recent years, a planar display (flat panel display), a typical example of which is a liquid crystal display device using the electro-optic property of liquid crystal, has been actively used.
As one form of the flat panel display, there is known an active matrix driving system wherein a TFT is fitted to each pixel and a picture is displayed by writing data signals successively. TFTs are essential elements for realizing the active matrix driving system.
In almost all cases, TFTs are manufactured using amorphous silicon. However, TFTs have low electric field effect mobility and cannot operate by frequencies necessary for processing picture signals. Therefore, TFTs are used only as switching elements fitted to respective pixels. A data line driving circuit for outputting picture signals to data lines or a scanning line driving circuit for outputting scanning signals to scanning lines is processed by an outside IC (IC driver) mounted by TAB (tape automated bonding) or COG (chip on glass).
However, as the density of pixels becomes larger, the pitch of the pixels becomes narrower; therefore, it is considered that the system in which a driver IC is mounted has a limit. For example, in the case that UXGA (pixel number: 1200×1600) is supposed, 6000 connecting terminals are required in an RGB coloring system at the simplest estimate. An increase in the number of the connecting terminals causes an increase in the probability that contact faults are generated. Moreover, the peripheral area (frame area) of its pixel section increases. As a result, it is unsuccessful that a semiconductor device using this as a display is made small-sized, and the design of the appearance thereof is damaged. In light of such background, a display device integrated with a driving circuit is clearly demanded. By integrating a pixel section with scanning line and data line driving circuits on a single substrate, the number of connecting terminals decreases drastically and the area of the frame area can also be made small.
As a means for realizing the above, suggested is a method of making TFTs of a polycrystalline silicon film. The field effect mobility of TFTs made of polycrystalline silicon film is higher than that of TFTs made of an amorphous silicon film, so that the TFTs can be operated by frequencies necessary to process picture signals. Therefore, using TFTs made of polycrystalline silicon film can be realized that the display device integrated with a driving circuit in which a pixel portion is integrated with scanning line and data line on the same substrate.
TFTs functioning as switching elements located in the pixel section are desired to have, as their characteristic, a small off-state current (Ioff). However, in the case that the TFTs are formed using polycrystal silicon, the TFTs have a problem that their off-state current becomes high because of defects formed in crystal boundaries.
TFTs are manufactured by using photo masks to etch a semiconductor film, an insulating film and/or a conductor film into a given shape while depositing these-films into a lamination. However, if the structure of the TFTs is made optimal to obtain desired characteristics of the switching elements in the pixel section, the number of the photo masks increases so that the manufacturing process of the TFTs becomes complicated. As a result, the number of the steps in the process increases inevitably.